1. Field of the Invention
The present invention relates to a method of controlling knocking in internal combustion engines equipped with a device for controlling the opening of inlet valves.
2. Description of the Related Art
An internal combustion engine with controlled ignition typically includes a number of cylinders, each of which has a piston sliding cyclically inside the cylinder and a spark plug which is cyclically driven by an electronic control unit to stroke a spark between its electrodes and thereby to cause the ignition of the compressed gas inside the cylinder.
The control unit comprises a memory that stores a series of maps that provide the values of driving points of the spark plugs as a function of the top-dead-center. In particular, for each spark plug the maps provide the value of the ignition advance, i.e. the value of the angle between the ignition (i.e. the spark between the spark plug's electrodes) and the top-dead-center or TDC of the piston. If the value of the ignition advance is zero, then the ignition, i.e. the spark between the spark plug's electrodes, occurs precisely at the top-dead-center of TDC or the piston.
Ignition advance values stored in maps contained in the control box are determined during the development of the engine in order to ensure good combustion under all possible operating conditions so as to have good thermal efficiency of the engine while, at the same time, safeguarding the integrity of the engine, i.e. avoiding the presence of excessive phenomena of knocking in the cylinders.
The knocking is an explosive type combustion of part of the air-fuel mixture that takes place before the air-fuel mixture is reached by the front of the flame generated by the spark plug. Following the knocking a series of pressure waves are created, and they travel through the combustion chamber and strike violently against the metal walls. The explosion occurs when specified critical temperature and pressure values inside the chamber are exceeded. These values can vary considerably from engine to engine. When the knocking takes place in the mid-bass systems, it often causes a typical metallic noise, clearly audible, known as “knocking”. When the knocking is of significant magnitude it is called “megaknock” and can have disastrous consequences for the integrity of the mechanical parts such as the piston.
The knocking occurs normally when the ignition advance is too great or when a fuel with too low octane is used. The power of an anti-knock fuel is precisely indicated by its octane rating. The knocking can also occur in turbocharged engines when the boost pressure is too high.
The evolution of combustion is influenced by many factors. Among the most important factors are the characteristics of the fuel, the temperature of the engine head, the degradation of the spark plugs whose effect is essentially impossible to predict with precision. Therefore it is necessary to use a knock sensor, which detects the presence of excessive knocking. In case of severe knocking in a cylinder, the control box shall reduce the value of the ignition advance for that cylinder so to eliminate the knocking in the cylinder. In this way the maximum cylinder pressure is reduced and is reached later than the TDCs, making the detonating event less likely.
However, in terms of combustion efficiency, reducing the ignition advance of a cylinder corresponds to a loss in thermodynamic efficiency: the mass of air going into the cylinder and thus the mass of fuel injected are kept constant, but by reducing the ignition advance the combustion efficiency is reduced, i.e. the fraction of chemical energy converted into mechanical energy. This has negative effects on fuel consumption and on the generation of pollutants.
Moreover, in a supercharged internal combustion engine knockings of high intensity might occur (commonly referred to as “megaknock”). Because of their violence, “megaknocks” are particularly dangerous for the integrity of the engine. In the particular case of “megaknock”, the pressure peaks that can be achieved within the cylinder are so high that even a few events can jeopardize the integrity of the engine. One of the characteristics of the “megaknock” is that it is an event comparable to a pre-ignition, or an independent self-ignition of the spark produced by the spark plug. However, this has the effect of decreasing the torque generated by the engine causing a “torque-hole” which is clearly noticeable by the driver with a clear deterioration in driving pleasure and comfort. In addition, this action is very slow because at high speeds, the effect of closing the throttle can be felt in the cylinders after a few engine cycles due to the inevitable inertia related to the distance between the throttle and the cylinders and due to the dynamics of emptying of the inlet manifold.